Iron Biominerals: An Overview

Abstract

INTRODUCfION Biomineralization processes, by which organisms form inorganic minerals, are broadly distributed and occur in almost every phylum of the biological world l ,2.3. There is a large diversity of minerals formed, with over 60 currently known l , The cations of the most widely occuring minerals are the divalent alkaline earths Mg, Ca, Sr and Ba. These are paired with the anions carbonate, hydroxide, oxalate, oxide, phosphate, sulfate and sulfide. Silica, hydrous silicon oxide, also occurs widely in algae (fable 1). These minerals function as exo-and endoskeletons, cation storage, lenses, gravity devices and in other roles in various organisms. IRON BIOMINERALS Minerals of iron are also known to occur in many organisms. This is due perhaps, to the important role of iron in many metabolic processes, and to the difficulty for organisms posed by the toxic products of ferrous iron oxidation by 02 together with the insolubility of ferric iron at neutral pH. Formation of iron minerals allows organisms to accumulate iron for future metabolic needs while avoiding high intracellular concentrations of ferrous iron. Other attributes of iron biominerals that are potentially useful to organisms include hardness, density and magnetism. An important class of iron biominerals are the ferric hydroxides or oxy-hydroxides, which occur as amorphous, colloidal precipiates, as quasi-crystalline minerals such as ferrihydrite, or as crystalline minerals such as lepidocrocite or goethite (fable 2). Amorphous iron oxy-hydroxides are found in the sheaths and stalks produced by the so-called iron bacteria, Leptothrix and Gallionella, which utilize the oxidation of ferrous ions by 02 as a source of Iron Biominerals. Edited by R. B. Frankel and R. P. Blakemore. Plenum Press. New York. 1990